An image forming apparatus forms an image using an exposure device including a plurality of light emitting elements, such as LEDs, arranged along a photoreceptor. The light emitted from the light emitting elements is focused on the photoreceptor through a lens array, and an electrostatic latent image is formed on a region of the photoreceptor at which the light is irradiated.
In such an image forming apparatus, as the light emitting elements and the lenses may have non-uniform optical characteristics and may be disposed non-uniformly, a light intensity distribution on the photoreceptor may be non-uniform. Thus, when a plain image, such as a half-tone image, is formed, a streak-shaped density unevenness (vertical streak and streak unevenness) may appear in the image.
One way to reduce the density unevenness is independently adjusting a light spot size formed on the photoreceptor by independently adjusting a current supplied to each of the light emitting elements. However, as a focal depth of the lens array is usually very narrow, the light intensity distribution is very sensitive to a positional change of the lens array and the light emitting elements. Thus, the adjustment of the light spot size needs to be performed with precision.